In the prior art there is no lack of proposals as to how the tool/machine calibration may be realized. In a first, very common method a tool height to center calibration (Z-direction) is performed by scribing a test part with the tool while the test part is prevented from rotation. Typically two lines are scribed, the first at a given angular position (B-angle), then a second line at a second fixed B-angle 180 degrees from the first B-angle. The distance between the two lines is measured with an optical microscope with an appropriate magnification and measurement reticule. The tool height is then manually adjusted by half the measured distance between the two lines, and the procedure is repeated until no separation between the lines can be observed. Finally a test lens is cut and the center is examined using an optical microscope. Small adjustments to the final calibration can be made at this stage.
The disadvantages with this first method are those of accuracy and repeatability being variable, and speed being slow and unpredictable. The speed and success of the whole procedure is typically dependent on operator experience and skill. Further, this is a tool height calibration only. The method does not lend itself to identifying the center and/or radius of the tool tip. This needs to be achieved using a different method. Also, another problem with this first method is possible damage to the tool during the scribing part of the procedure. Finally, this is only a partial tool calibration, offering Z-height only, and still requires final test piece verification/adjustment using an optical microscope.
A second method as disclosed in, e.g., the “NANOFORM® SERIES OPERATOR'S MANUAL” of Precitech Inc., Keene, N.H., USA, uses a special camera accurately positioned relative to the spindle of the machine. The optical axis of the camera is generally parallel to the Z-axis. The camera is mounted at a known and repeatable position in all three (X, Y, and Z) directions relative to the machine spindle (headstock), typically using a kinematic coupling interface to allow for quick insertion and removal of the camera into/from the machine. The camera optics are typically using a very short focal depth of field, and the position of this focal plane needs to have been previously pre-adjusted and fixed in order to perfectly coincide with the center of the spindle rotation axis (Z-height). The camera's image is electronically displayed on a computer monitor or other suitable output device to allow for viewing by the operator. The camera optics are adjusted and fixed so the camera's focus (on the tool's rake face) is used to adjust the Z-height of the tool relative to the axis of rotation. The tool height is manually adjusted by the operator by turning an adjustment screw until the tool is brought into focus. This provides a preliminary tool height (Z) calibration. At this point, the operator can move the tool relative to the image using his X, Y jog capability, and visually aligns three different points on the edge of the tool with the cross hairs of the imaging system. These points are captured numerically by the computer system, and used to calculate a best fit circle corresponding to the cutting edge of the tool.
The tool height obtained with focus was said to be a preliminary height (Z) adjustment only. As a final step to obtain a good tool height calibration, a rotationally symmetrical test piece is cut, and its center is observed by the operator using an optical microscope. Depending on what is observed at the center of this test piece a corresponding adjustment is made to the tool height. This final test piece cutting and observation procedure normally needs to be repeated until the operator is satisfied he has achieved a good calibration.
The disadvantages with this approach are those of speed, and operator involvement. Also, unless many hundreds of points along the tool edge are captured at sub-micron accuracies, which is not practical at all, the method cannot automatically calibrate for tool tip circularity errors. Standard practice therefore typically involves purchasing of more expensive “controlled waviness” tools, i.e. very precise tools with low deviation from the best fit circle.
Another problem with this approach is identified when the tool tip has a “blunt edge”. Blunt edge tools are used in special cases where certain types of material respond better to high negative rake situations. In these cases it is common to use a slightly chamfered or radiused edge treatment so that the actual cutting point of the tool tip can be located many microns below the rake face of the tool. In this case, measuring the height of the tool using a focus point on the rake face does not properly identify the height of the true point at which the tool cuts; and accurately focussing at the very edge is quite difficult.
Again, the second method is only a partial calibration since it does not calibrate for circularity errors, and also requires final test piece verification/adjustment using an optical microscope.
Other optical based methods and apparatus used to do a tool/machine calibration are described in documents U.S. Pat. No. 5,825,017 and U.S. Pat. No. 4,656,896. These methods, however, have the same disadvantages as described above.
A third method uses touch probes to probe the tool in different directions, either on or off the machine. Different documents describe mechanisms and variations of this approach, including U.S. Pat. No. 5,035,554, U.S. Pat. No. 4,417,490, U.S. Pat. No. 4,083,272 and U.S. Pat. No. 4,016,784. However, none of these methods calibrate for tool tip radius, or circularity. In addition, like was the situation with the second method, tool height cannot be accurately determined if the tool has a blunt edge since only the rake face is mechanically probed.
Applicable to all the above methods is a procedure commonly used to improve the form accuracy of precision optical surfaces. This method is described in literature from Moore Nanotechnology Systems, LLC, Keene, N.H., USA, regarding a “Workpiece Measurement & Error Compensation System (WECS™)”, and again Precitech Inc., Keene, N.H., USA, concerning the “ULTRACOMP™ Form Measurement & Error Compensation System”. This technology is typically a “part dependent” error measurement and compensation procedure, and as such it is applied to only one part geometry at a time. By this it is meant that after a part is cut, the errors are measured on that part, and then error compensation is applied when the part is recut. If a different part, with different geometry is cut, the full procedure is repeated for the new part. This means it is not a general machine calibration meant to be used on any geometry, but is rather geometry specific.
This procedure has the disadvantage that it is slow and time consuming to apply, due to the fact that it needs to be repeated for each part geometry to be cut. Also, this method only maps errors on one side of center, meaning it does not consider the possibility of cutting parts with prism, i.e. parts having a surface which is tilted with respect to the axis of rotation. Thirdly it is not a calibration method which lends itself to a general tool/machine calibration including Z-height errors. The machine needs to be pre-calibrated and cutting accurately to center before this method can be implemented.
Summarily, the current state of the art uses methods which are based on manual, operator dependent procedures, and are therefore prone to errors, provide for partial tool calibration only and/or are slow in their implementation and practice.
Therefore, what is needed is a method for auto-calibration of a tool in a single point turning machine used for manufacturing in particular ophthalmic lenses, by which two-dimensional (2D) tool/machine calibration and three-dimensional (3D) tool/machine calibration, respectively, can be performed in a reliable and economic manner.